Liquid phase oxidation of cycloalkane compounds plays an important role in the petrochemical industries. Among various liquid phase oxidation reactions, oxidation of cyclohexane is a considerably important route to produce cyclohexanol, cyclohexanone and adipic acid, whereby cyclohexanol and cyclohexanone can further react to form caprolactam. Moreover, caprolactam can undergo decyclization and polymerization to produce Nylon 6, while adipic acid and hexylene diamine are condensed and polymerized to produce Nylon 66.
Currently, cyclohexane and phenol are used as raw materials for producing cyclohexanone and caprolactam. As preparation of cyclohexanone by using phenol added with hydrogen is relatively higher in cost, conventionally cyclohexane is used as a reactant and charged with air to be oxidized under high temperature and high pressure conditions. This oxidation reaction firstly forms a hexyl hydroperoxide as an intermediate which then decomposes to be cyclohexanone and cyclohexanol customarily referred to as KA oil, wherein cyclohexanol further needs to be dehydrogenated to form cyclohexanone, and cyclohexanone after undergoing amination is subject to the Beckmann rearrangement reaction to produce caprolactam with the use of sulfuric acid as a catalyst.
In general, conventionally oxidation of cyclohexane is performed at a temperature between 140° C. to 170° C. and under an air pressure of 5 to 25 bar or a higher pressure. This cyclohexane oxidation is carried out by air to produce a hexyl hydroperoxide (intermediate), cyclohexanol and cyclohexanone with or without using a catalyst. However, conversion rate of cyclohexane oxidation is not high, normally below 10%. For example, as reported in Taiwanese Patent Publication No. 150309, target products, cyclohexanone and cyclohexanol, from oxidation of cyclohexane are low oxidation state compounds and easily further react to form high oxidation state compounds. As a result, by such a conventional cyclohexane oxidation process, selectivity of cyclohexanone and cyclohexanol is not good, and the low conversion oxidation reaction would primarily reduce acids or other side products produced from over oxidation. Therefore, it is critical to control product selectivity in order to improve the conventional oxidation process; besides, improvement in conversion rate can help reduce load of product separation and increase unit space-time-yield, thereby enhancing reaction rate of the oxidation process.
Moreover, in the petrochemical industries, conversion rate and selectivity of the foregoing cyclohexane oxidation process can normally be improved by addition of catalysts. In a reaction of liquid phase oxidation of carbohydrates, it usually uses transition metal ions or complexes thereof as homogenous catalysts to enhance reaction rate. These transition metal ions can be Co2+, Cr2+, Mn2+, Cu2+, Ni2+ and the like, wherein Co2+ has the most preferable activity. For example, U.S. Pat. No. 3,987,100 discloses oxidation of cyclohexane performed by air with addition of Co2+ and Cr2+ as binary catalysts (0.1-5 ppm Co2+ and 0.02-0.9 ppm Cr2+). U.S. Pat. No. 4,658,056 teaches the use of a binary catalyst of combining bi[di(2-ethylhexyl)phosphoric acid]cobalt and cycloalkanoic acid chromium for performing oxidation with air under high temperature and high pressure conditions. In U.S. Pat. No. 2,851,496, cyclohexane oxidation is carried out with air at a reaction temperature of 160° C. and a reaction pressure of 160 psi, and products obtained from oxidation are allowed to contact with a sintered cobalt oxide fixed bed reactor at 70° C. for 15 minutes to improve conversion rate and selectivity of the cyclohexane oxidation reaction. Luna et al (“Cyclohexane Oxidation Using Transition Metal-Containing Aluminophosphates (MAPO-VFI)”, Journal of Molecular Catalysis, vol. 117, pp. 405-411) propose the use of MAPO-VFI as a catalyst in the cyclohexane oxidation reaction, which can increase a ketone alcohol ratio to be 15.3; however, the reaction time for this oxidation reaction is longer than 24 hours, and an amount of catalyst as high as 1% is required, thereby not suitably applied to large scale production in the industry.
In another aspect, conversion rate and selectivity of cyclohexane oxidation can also be improved by modifying a gas-liquid contact way for an introduced gas. For example, in U.S. Pat. No. 3,957,876, a distilling tower reactor is used to allow fed cyclohexane to flow downwardly and effectively mix with upwardly moving air distributed in different layers, wherein each layer is provided with an oxygen removing layer used to maintain concentration of incompletely reacted oxygen below 4% from liquid space to vapor space in the distilling tower so as not to form explosive gaseous mixtures. U.S. Pat. No. 6,075,169 discloses the use of gas-liquid countercurrent to enhance mass transfer effect between air and liquid cyclohexane. However, these patents fail to effectively increase a ketone alcohol ratio for the cyclohexane oxidation process.
Besides, conversion rate and selectivity of the cyclohexane oxidation process can also be improved by introduction of an oxygen enriched gas (containing more than 21% oxygen) or pure oxygen. However, this method of using the oxygen enriched gas or pure oxygen may induce potential deflagration and thus is hardly applied to the industry. Deflagration is a type of burning reaction; mixtures of inflammable gases and oxidants such as oxygen are burned and lead to significant increase in temperature and pressure, which is customarily referred to as deflagration or explosion. Deflagration is induced by complete oxidation, instead of incomplete or partial oxidation, incurred in the oxidation process, which may render serious risks as the reaction exceeds explosion limits, whereby the pressure would instantly raise and possibly causes the reactor to explode.
U.S. Pat. Nos. 5,780,683 and 6,008,415 propose the use of a special liquid phase oxidation reactor in which an oxygen enriched gas or pure oxygen is charged and used for oxidation of cyclohexane; special downwardly stirring blades are adopted to mix cyclohexane and the oxygen enriched gas, and an enclosure is used to effectively block and consume oxygen, allowing incompletely reacted oxygen in a small amount to be deactivated by nitrogen when passing from liquid space to vapor space. Compared to conventional oxidation in the use of air, with the same cyclohexane conversion of 4%, the above oxidation process by using the oxygen enriched gas or pure oxygen reduces the reaction temperature from 160° C. to 149° C. and reduces reaction residence time from 36 minutes to 8 minutes, as well as increases space-time-yield from 0.45 gmol/hr·L to 1.85 gmol/hr·L and increases a ratio of cyclohexanone to cyclohexanol from 0.48 to 0.77. Therefore, in compliance with safety requirements, the cyclohexane oxidation process can be performed under the same reaction conditions but with increased oxygen concentration of the oxygen enriched gas to improve yield and productivity thereof. However, Williams et al. (“Developing Safe Oxygen-Based Commercial Liquid-Phase Oxidation Reactors” AlChE Annual Meeting, Miami, Fla., 1998) has reported that cyclohexane oxidation performed by using oxygen enriched gases or pure oxygen may exceed explosion limits of cyclohexane and lead to potential risks e.g. deflagration as bubbles of oxygen enriched gases or pure oxygen contain cyclohexane vapor; this should be of significant concerns in respect of large scale production in the industry. In addition, controls of oxygen dispersion and stirring play an important role in this oxidation process; if stirring is not effective, bubbles in liquid phase would coagulate to form partial and potentially explosive vapor space, such that the oxidation process needs to be carefully operated in practice.